The present invention relates to a silicone grease composition or, more particularly, to a silicone grease composition having an improved characteristic of extreme-pressure lubrication in steel-to-steel friction.
As is well known, silicone oils, i.e. organopolysiloxane fluids, have excellent properties including very low temperature dependency of viscosity, high resistance against heat and oxidation, high stability under shearing, chemical stability and so on. These properties are important factors in lubricating oils so that silicone oils are widely used as a lubricating oil in some applications. Similarly, silicone greases prepared from a silicone oil as the base oil also have very low temperature dependency of the consistency and high resistance against heat and oxidation as well as high chemical stability.
Silicone greases, of course, are not free from some problems, of which the largest is the poor boundary lubrication for steel-to-steel friction in comparison with greases of other types prepared from a mineral oil or synthetic oil as the base oil. Therefore, no satisfactory lubrication can be obtained with a silicone grease for friction under high-speed sliding or under high contacting pressure so that the fields of application of silicone greases are limited in this regard.
Various attempts and proposals have been hitherto made to improve the boundary lubrication of silicone greases by admixing the grease or the silicone oil as the base oil with certain additives called an oiliness improver or extreme-pressure additive. Such additives hitherto proposed include fatty acids and derivatives thereof, chlorinated and fluorinated organic compounds, organic compounds of boron, lead, manganese, tin and the like, and so on. For example, Japanese Patent Publication No. 51-38864 teaches admixture of a silicone grease with a chlorinated paraffin or a dialkyl chlorendate as a class of chlorinated organic compounds. These chlorine-containing additives are indeed effective in improving the boundary lubrication of silicone greases but are not practical due to the very disadvantageous corrosiveness against metals at elevated temperatures prohibiting the use of such a silicone grease at a high temperature or under an oxidizing condition.